1. Field
The present invention relates generally to a communication system. The present invention relates in particular to a transmitter for use in the communication system, a method of transmitting control data and information data in the communication system, and a remote station for use in the communication system.
2. Background
Modern mobile cellular telephones are able to provide conventional voice calls and data calls. The demand for both types of calls continues to increase, placing increasing demands on network capacity. Network operators address this demand by increasing their capacity. This is achieved, for example, by dividing or adding cells and hence adding more base stations, which increases hardware costs. It is desirable to increase network capacity without unduly increasing hardware costs, in particular to cope with unusually large peak demand during major events such as an international football match or a major festival, in which many users or subscribers who are located within a small area wish to access the network at one time.
When a first remote station is allocated a channel for communication, a second remote station can only use the allocated channel after the first remote station has finished using the channel. Maximum cell capacity is reached when all the allocated channels are used in the cell. This means that any additional remote station user will not be able to get service. Co-channel interference (CCI) and adjacent channel interference (ACI) further limit network capacity and will be discussed below.
Network operators have addressed this problem in a number of ways, all of which use added resources and added cost. For example, one approach is to divide cells into sectors by using sectored, or directional, antenna arrays. Each sector can provide communications for a subset of remote stations within the cell and the interference between remote stations in different sectors is less than if the cell were not divided into sectors. Another approach is to divide cells into smaller cells, each new smaller cell having a base station. Both these approaches are expensive to implement due to added network equipment. In addition, adding cells or dividing cells into smaller cells can result in remote stations within one cell experiencing more CCI and ACI interference from neighboring cells because the distance between cells is reduced.
According to another approach, a base station 110, 111, 114 may transmit two signals on the same channel, each signal for one of two users, by operating according to methods known collectively as either Multi-User on One Slot (MUROS) or Voice services over Adaptive Multi-user on One timeSlot (VAMOS). According to the methods, a different training sequence is used for each signal.
One remote station may receive its own wanted SACCH data and unwanted SACCH data for another remote station contemporaneously on the same channel. If the one remote station receives the unwanted SACCH data at a higher power level than the level at which it receives its own wanted SACCH data, say 10 dB higher, then the unwanted SACCH data may interfere with the wanted SACCH data so that the quality of the received wanted SACCH data is degraded too much for a call to be maintained by the one remote station.
Pending international patent application having application number PCT/US2008/085569, filed on Dec. 4, 2008 and assigned to the assignee hereof, describes that newer codecs such as AMR allow lower bit rate modes to be used for channels experiencing poor radio channel conditions. There is generally no such mechanism of adjusting bit rate for signaling channels (e.g. SACCH) and therefore the signaling data is less well protected against channel degradations than traffic data. SACCH data is affected worse by co-channel operation than traffic (TCH) data because the SACCH has no redundancy, i.e., every SACCH frame must be received with few errors.
DTX is a method that improves overall efficiency of a wireless device by momentarily discontinuing the transmission of voice data when there is no significant voice input to the microphone of the wireless device (e.g. a remote station). Typically in a two-way conversation, a user of a remote station speaks during slightly less than half of the time. The duty cycle of the transmission can be cut to less than 50 percent if the transmitter signal is switched on only during periods of voice input. This improves efficiency by reducing interference and by conserving battery power.
An ongoing voice call is maintained by messaging on the slow associated control channel (SACCH). The SACCH is transmitted once during every SACCH period. DTX is operated during speech frames. The SACCH signaling frame does not use this DTX mode. That is to say, the SACCH may not get benefit from DTX in the same way that TCH does get benefit from DTX. The interference of the SACCH for a first of two paired remote stations is continuously present at the receiver of the second paired remote station.
There is therefore a need to provide improved protection of interference-sensitive data intended for a particular receiver against other interfering data not intended for the particular receiver.